Suppose I use grml-rescueboot on Debian/Ubuntu to boot up an MX18 iso, would I be able to install MX18 to my harddrive, assuming I don't overwrite the /boot partition from which my MX18 iso is being grml-rescuebooted?

apt show grml-rescueboot
...
Description: Integrates Grml ISO booting into GRUB
This package provides a script for update-grub which looks for
Grml ISO images in /boot/grml and automatically adds an entry
for each image. The purpose is to use one of those images to
boot a Grml rescue system without using a CD or USB stick.

I would be surprised if this booted correctly. Please let us know if it does. Why don't you want to boot MX the normal way? Do you have no usb stick to use? If you are on Linux and have "git" installed and you have one of our iso files and a usb-stick then you can make a full featured live-usb with these simple steps:

Almost a decade ago, I started working on our live system to make it easy for me to add Zsh to antiX live so I could have a fast-booting rescue usb that ran Zsh. It sounds like antiX is closer to grml than MX is. It has very good command line support including our cli control centre: antiX-cli-cc. Both antiX-base and antiX-full contain Fluxbox and several other lightweight window managers. OTOH MX provides a smoother GUI experience with a highly integrated XFCE desktop. The MX community repos make it shine and set it apart from most other Debian derivates. You literally get the best of both worlds (stable and testing). Both antiX and MX use the same live system.

Thank you although for me that does not work yet with an error of something like, "Can not find /ISO/MX-18_x64.iso." I believe this is because most of my hdd is encrypted. I have an unencrypted /dev/sda1 mounted at /boot, and an encrypted /dev/sda2 containing an lvm group with two volumes, / and swap. It would make sense if I were able to do something like put the ISO directory on my unencrypted /boot partition and point the custom.cfg to that, but I have similar error with that, too.

this is because most of my hdd is encrypted.
It would make sense if I were able to do something like put the ISO directory on my unencrypted /boot
partition and point the custom.cfg to that, but I have similar error with that, too.

ohh ... nice you let us know and was certainly not tested with encryption in mind but can surely be done too.
What about showing us quick system info (aka "inxi -Fxz" )

I just plopped the ISO right in /boot (/boot/MX-18_x64.iso). I specify the unencrypted drive (referred to as (hd0,1), aka /dev/sda1 in grub terms) as iso_path to be used for loopback. However, in my kernel params, I need to specify the iso file only for my fromiso value. Now... I wonder if I can install to hdd from this...

I just plopped the ISO right in /boot (/boot/MX-18_x64.iso). I specify the unencrypted drive (referred to as (hd0,1), aka /dev/sda1 in grub terms) as iso_path to be used for loopback. However, in my kernel params, I need to specify the iso file only for my fromiso value. Now... I wonder if I can install to hdd from this...

A small issue in the custom.cfg you posted the "iso_path" need to be the path without the device,
otherwise the "buuid" might not get set properly.
So this shall work:

And yes you can install, with two potential restrictions:
do install to an empty ( or better) pre-exstings partiton as (non-enrypted).
Do not install grub to the MBR but rather to "root",
in order to avoid to overwrite your existing grub (with encryption)
Boot into the encrypted install and run within the encrypted installation